The mechanisms of action, the regulation and the roles in intermediary metabolism of the enzymes involved in the conversion of thymidine to the pyrimidine nucleotides of RNA in Neurospora crassa are being studied. The enzymes which are being purified and subjected to mechanistic studies include those that catalyze the reactions of the following pathway: pyrimidine deoxyribonucleoside yield 1 ribonucleoside; ribonucleoside yield 2 pyrimidine plus ribose; thymine yield 3 5-hydroxymethyluracil yield 4 5-formyluracil yield 5 uracil-5- carboxylic acid yield 6 uracil plus CO2. The studies involving the alpha-ketoglutarate dioxygenase reactions, e.e., 1,3-5, are being emphasized.